Glass frit wafer bonding process and packages formed thereby

ABSTRACT

A method of glass frit bonding wafers to form a package, in which the width of the glass bond line between the wafers is minimized to reduce package size. The method entails the use of a glass frit material containing a particulate filler material that establishes the stand-off distance between wafers, instead of relying on discrete structural features on one of the wafers dedicated to this function. In addition, the amount of glass frit material used to form the glass bond line between wafers is reduced to such levels as to reduce the width of the glass bond line, allowing the overall size of the package to be minimized. To accommodate the variability associated with screening processes when low volume lines of paste are printed, the invention further entails the use of storage regions defined by walls adjacent the glass bond line to accommodate excess glass frit material without significantly increasing the width of the bond line. The storage regions also ensure adequate flow of glass frit material around electrical runners that cross the glass bond line, as well as into any isolation trenches surrounding the runners.

TECHNICAL FIELD

The present invention generally relates to wafer bonding methods. Moreparticularly, this invention relates to a wafer bonding process thatuses a glass frit bonding material to bond a pair of wafers and form apackage with an enclosed cavity, wherein the stand-off distance betweenthe wafers is controlled by the bonding material and the width of thebonding surface contacted by the bonding material is minimized to reducepackage size.

BACKGROUND OF THE INVENTION

Within the semiconductor industry, there are numerous applications thatrequire bonding a semiconductor wafer to a second wafer or glasssubstrate. As an example, a microelectromechanical system (MEMS) deviceformed in or on a semiconductor wafer (referred to herein as a devicewafer) is often capped by a semiconductor or glass wafer (referred toherein as a capping wafer), forming a package that defines a cavitywithin which the MEMS device is enclosed and protected. Examples of MEMSdevices protected in this manner include accelerometers, rate sensors,actuators, pressure sensors, etc. By the very nature of their operation,MEMS devices must be free to move to some degree, necessitating that theseal between the wafers is sufficient to exclude foreign matter from thecavity. Certain MEMS devices, such as absolute pressure sensors, furtherrequire that the cavity be evacuated and hermetically sealed. Theperformance of motion sensors with resonating micromachined componentsalso generally benefit if the cavity is evacuated so that themicromachined components operate in a vacuum. A hermetical seal alsoensures that moisture is excluded from the cavity, which might otherwiseform ice crystals at low temperatures that could impede motion of theMEMS device.

In view of the above, the integrity of the bond that secures the cappingwafer to the device wafer is essential to the performance and life ofthe enclosed MEMS device. Various bonding techniques have been used forthe purpose of maximizing the strength and reliability of the waferbond, as well as various intermediate bonding materials, includingadhesives, solders, and dielectrics such as glass frit. Silicon directand anodic bonding techniques that do not require an intermediatematerial have also been used. As would be expected, each of thesebonding techniques can be incompatible or less than ideal for certainapplications. Silicon direct and anodic bonding methods require verysmooth bonding surfaces, and therefore cannot produce a vacuum seal whentrench isolation or unplanarized metal crossunders are employed on thedevice wafer, such as to electrically interconnect a MEMS device to bondpads outside the vacuum-sealed cavity of the package. In contrast,intermediate bonding materials such as glass frit are able to formsuitable bonds with deposited layers, runners and other surfacediscontinuities often found on device wafers.

Glass frit bonding materials used for wafer bonding are often depositedby a screen printing technique, in which case the material is depositedas a paste that contains a glass fit, a thixotropic binder, and asolvent for the binder. The proportions of glass frit, binder andsolvent are adjusted to allow screen printing of a controlled volume ofthe paste on a designated bonding surface of one of the wafers,typically on the capping wafer. After firing to remove the binder andsolvent the capping and device wafers are aligned and then mated so thatthe remaining glass flit particles (bonded together as a result of thefiring operation) contact a complementary bonding surface of the second(e.g., device) wafer. The wafers are then heated to melt the glass frit(e.g., about 425° C.), so that on cooling the glass frit materialresolidifles to form a substantially homogeneous glass bond line betweenthe wafers.

While a certain bond line width is necessary to form a sufficientlystrong wafer bond, minimizing the width of the bond line is desirablefrom the standpoint of reducing the chip size, which in turn enables themaximum number of chips to be fabricated on a wafer slice. The minimumwidth and volume of a screen printed glass bond line is not typicallylimited by concerns for bond strength, but by the capability of thescreen printing process. Because of an unacceptable variability ofscreening processes when thin and narrow lines of paste are printed, thevolume of glass frit paste printed is typically greater than thatrequired to effect a reliable hermetic wafer bond. To control the“stand-off” distance between wafers, the final thickness of the glassbond line may be established by “stand-offs” micromachined in one of thewafers. When the capping and device wafers are mated, pressure isapplied to bring the stand-offs into contact with the surface of thedevice wafer, thus physically establishing the wafer spacing.Consequently, both wafers must have surfaces dedicated to accommodatingthe stand-offs, increasing the chip size. The excess bond material isforced outward relative to the original printed bond line, leading to arelatively wide bond line that must be accommodated by the respectivebonding surfaces on the wafers. As a result, relatively wide bond linesand micromachined stand-offs associated with current glass bondingtechniques have artificially limited the size to which wafer bonded chippackages can be reduced.

In view of the above, it would be desirable if an improved wafer bondingprocess were available that could reduce the widths of the wafer bondingsurfaces in order to maximize the chip multiple per wafer slice. Itwould be further desirable if such a process could simplify waferfabrication while reducing package cost.

SUMMARY OF THE INVENTION

The present invention provides a method for glass frit bonding wafers toform a package, in which the width of the glass bond line is minimizedto reduce package size. A particular example is the capping of a devicewafer to enclose a micromachined sensing structure, such as a MEMSdevice. The invention entails the use of a glass frit material thatestablishes the stand-off distance between wafers, instead of relying ondiscrete structural features on one of the wafers dedicated to thisfunction. In addition, the amount of glass frit material used to formthe glass bond line between wafers is reduced to such levels as toreduce the bond line width, allowing the overall size of the package tobe minimized. To accommodate the variability associated with screeningprocesses when low volume lines of paste are printed, the inventionfurther entails the use of storage cavities adjacent the bond line toaccommodate excess glass frit material without significantly increasingthe width of the bond line.

The method of this invention generally entails providing a paste thatcomprises, in addition to a glass frit material for the glass bondingprocess, a particulate filler material having a higher meltingtemperature than the glass frit material and a diametrical dimensioncorresponding to the stand-off distance desired between the wafers to bebonded. The paste is deposited on a bonding surface of a first wafer soas to define a bond line thereon, and the first wafer is then heated atleast sufficiently to remove any volatile constituents of the paste. Thefirst wafer is then mated with a second wafer so that the bond line isbetween and contacts bonding surfaces of both wafers. As a result of thediametrical dimension of the particulate filler material, the stand-offdistance between the bonding surfaces of the wafers is approximatelyequal to the diametrical dimension of the filler material. The wafersare then sufficiently heated to melt the glass frit material but not thefiller material, and then cooled to form a glass bond line between thewafers, at which time the bonding surfaces of the wafers remainapproximately spaced apart by the stand-off distance established by thefiller material.

According to a preferred aspect of the invention, at least one storagearea defined by a cavity, trench, etc., is present in the surface of thefirst wafer, and the paste is printed on the bonding surface of thefirst wafer adjacent the storage area so that any excess portion of thepaste flows into the storage area during the mating step. The paste isalso preferably printed adjacent a peripheral edge of the first wafer,so that any additional excess portion of the paste flows beyond theperipheral edge when the wafers are mated together. The storage area andperipheral edge are preferably defined by respective walls contiguousbut not perpendicular to the bonding surface of the first wafer. As aresult, the portions of the paste that flow into the storage area andbeyond the peripheral edge flow along the sloping walls away from thesecond die, such that these portions have a relatively large combinedvolume, preferably greater than the volume of the remainder of the pasteremaining between the bonding surfaces, the storage area and theperipheral edge.

Using the method of this invention, the width of a glass bond line canbe significantly reduced, thereby reducing the width of the bondingsurfaces required to accommodate the bond line. In addition, thefabrication of a device package can be simplified by eliminating theneed for micromachined structures to physically establish the stand-offbetween wafers. As a result, the package size can be reduced and thenumber of packages that can be fabricated on a given wafer increased.Though minimizing the width of the bond line, the present inventionenables a sufficient amount of bonding material to be available to fillany trenches or other surface discontinuities present in the surface ofthe wafers, while any excess bonding material is accommodated whilehaving minimal impact on the bond line width. Accordingly, the waferbonding process of the present invention is able to yield a highlyreliable package containing a MEMS device that can be hermeticallysealed within the package, while also reducing the cost of the package.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an edge of a MEMS device packagethat includes a capping wafer glass frit bonded to a device wafer inaccordance with this invention.

FIGS. 2 and 3 are plan views of the device and capping wafers,respectively, of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 represents a portion of a MEMS device package 10 that has beenglass frit bonded in accordance with this invention. The package 10 isformed by bonding a device wafer 12 to a capping wafer 14, such that amicromachined structure 16 (schematically represented in FIG. 2) isenclosed within a cavity 18 between the wafers 12 and 14. The wafers 12and 14 are preferably silicon, and the device wafer 12 preferablymonocrystallographic silicon, though it is foreseeable that othermaterials could be used. For example, the capping wafer 14 can be formedof glass, ceramic, or a semiconducting material other than silicon. Themicromachined structure 16 can be of any desired type, such as a proofmass, resonating structure, diaphragm or cantilever that relies oncapacitive, piezoresistive and piezoelectric sensing elements to senseacceleration, motion, pressure, etc., all of which are known in the art.A particularly notable example is a MEMS device disclosed in U.S. patentapplication Ser. No. [Attorney Docket No. H-203587] to Rich, assigned tothe assignee of this invention and incorporated herein by reference.

As is conventional, the micromachined structure 16 is electricallyinterconnected to metal bond pads 20 on the device wafer 12 byconductive runners 22. Through the bond pads 20, the micromachinedstructure 16 and its associated sensing elements can be electricallyinterconnected with appropriate signal conditioning circuitry (notshown), which may be formed on the device wafer 12, the capping wafer 14or a separate chip. The runners 22 are represented in FIG. 2 as beingisolated by trenches 24 from the surrounding substrate surface of thedevice wafer 12. As also shown in FIG. 2, the runners 22 and theircorresponding trenches 24 cross through a bonding surface 26 contactedby a glass bond line 30 (FIG. 1) that bonds a corresponding bondingsurface 28 of the capping wafer 14 to the bonding surface 26 of thedevice wafer 12. The distance between the bonding surfaces 26 and 28 ofthe wafers 10 and 12 is referred to herein as the stand-off height, andis important from the perspective of providing adequate clearance and,if applicable, damping control for the micromachined structure 16 withinthe cavity 18, as well as to establish a predetermined space between thewafers 10 and 12 that can be filled with a substantially consistentamount of glass among the packages singulated from a single wafer.

In FIGS. 1 and 3, the capping wafer 14 is portrayed as being etched orotherwise fabricated to have a peripheral edge 32 and a trench 34separated by the bonding surface 28. The trench 34 is spaced inwardly apreferably uniform distance from the peripheral edge 32, and iscontinuous so that the bonding surface 28 is also continuous andcomplementary to the bonding surface 26 of the device wafer 12. Forreasons to be explained below, the edge 32 and trench 34 of the cappingwafer 14 are defined by respective walls 36 and 38 that are contiguouswith the bonding surface 28 of the capping wafer 14. The walls 36 and 38are shown as being oblique to the bonding surface 28 of the cappingwafer 14. In one example, the walls 36 and 38 are fabricated to bedisposed at an angle of about 54.7 degrees to the bonding surface 28 byforming the capping wafer 14 of 1,0,0 silicon, and anisotropicallyetching the peripheral edge 32 and trench 34 with an appropriateetchant, suitable examples of which include alkali-type wet etchantssuch as potassium hydroxide (KOH) and tetramethyl ammonium hydroxide(TMAH), and pyrocatechol ethylenediamine. However, for reasons to becomeapparent, it is believed that suitable results could be obtained if thewalls 36 and 38 are sloped at greater or lesser angles, includingperpendicular to the bonding surface 28, through the use of differentwafer materials and etchants.

According to prior practice, the bond line 30 is preferably formed witha paste composition that includes a glass frit material, a thixotropicbinder and a solvent, in which the binder and solvent are removed andthe glass frit material is melted by sufficiently heating the paste,with the result that essentially only glass remains as the bond line 30.Suitable binders and solvents for the paste are those used incommercially-available glass frit paste mixtures, such as acrylics forthe binder. Suitable solvents and binders generally have vaporizationtemperatures on the order of about 100° C. and about 300° C.,respectively, and therefore well below the softening point of glass fritmaterials suitable for the paste (e.g., about 355° C.). As known in theart, the proportions of the glass frit material, binder and solvent arechosen at least on part to achieve the desired deposition resolution forthe paste, and consequently, the minimum width and thickness of theglass bond line 30. A preferred deposition method is screen printing inaccordance with known practices, by which the paste is deposited througha mask or screen to form a thick film on the bonding surface 26 or 28 ofone of the device or capping wafers 10 or 12. The paste is typicallyloaded onto the mask, and a squeegee blade is drawn across the surfaceof the mask to press the paste through apertures onto the bondingsurface 26 or 28. Thick films can be accurately printed by screenprinting to dimensions of about 0.3 mm, with increased variability inwidth and volume of the thick film typically occurring with decreasingfilm widths.

According to the invention, the paste is formulated to further include aparticulate filler material that has a higher melting temperature thanthat of the glass frit material. A particularly suitable composition forthe filler material is cordierite, though silicon or otherhigher-melting temperature materials could be used. The size range of afiller material is chosen to have a diametrical size approximately equalto the stand-off distance desired between the device and capping wafers10 and 12. In a preferred embodiment, the particle size range of thefiller material is about four to about six micrometers which, when thepreferred bonding process of this invention is employed (as describedbelow), yields a stand-off distance typically in the range of about fiveto about seven micrometers. The composition and size of the glass fritmaterial can be chosen on the basis of other process and compositionalconsiderations, including screening properties, process temperatures,etc. Suitable glass frit materials include borosilicate glasses, thoughit is foreseeable that the glass frit could be a mixture of variousoxides, such as litharge (PbO; also known as lead oxide, yellow and leadmonoxide), boric acid (H₃BO₃) which serves as a source for boron oxide(B₂O₃), silicon dioxide (SiO₂; silica), aluminum oxide (Al₂O₃, alumina),titanium oxide (TiO₂, titania), cupric oxide (CuO)₃, manganese dioxide(MnO₂) or manganese carbonate (MnCO₃) as a source for manganous oxide(MnO), calcia (CaO), lithium oxide (Li₂O), ceria (CeO₂), cobaltouscarbonate (CoCO₃), and others.

In an exemplary process in accordance with this invention, a glass fritpaste is screen printed on the bonding surface 28 of the capping wafer14 so that the paste defines a continuous bond line between the edge 32and trench 34 of the capping wafer 14. If the width of the bondingsurfaces 26 and 28 is about 300 micrometers, a suitable width for thepaste printed on the bonding surface 28 of the capping wafer 14 is about250 micrometers. In the preferred embodiment, the capping wafer 14 isthen heated to remove the solvent and binder from the paste, andthereafter melt the glass frit material so that the bond line nowcomprises the filler material dispersed in a molten glass material.Heating the wafer 14 to remove the binder and solvent and melt the glassfrit material can be performed in separate steps. For example, thebinder and solvent can be sequentially removed by firing in an airatmosphere, after which the glass frit material is flowed in a vacuum.In one example, the wafer 14 is baked at about 100° C. to remove thesolvent, then baked at about 300° C. to remove the binder, followed by abake at a temperature of about 340° C. to soften or melt the surfaces ofthe glass particles, effectively bonding or “tacking” the particlestogether. The preceding steps may be performed in air, after which theoven is evacuated and the wafer 14 is baked at about 420° C. tocompletely melt the glass frit material. Though slight devitrificationof the glass material during the 420° C. bake may occur, raising theuseful softening point of the glass (e.g., from about 355° C. to about385° C.), this intermediate glass reflow step allows the capping wafer14 to be handled upon cooling without any significant risk of glassparticles being dislodged from the bonding surface 28 and ending upelsewhere on the wafers 12 and 14.

After the glass bonding material has resolidified, the capping anddevice wafers 12 and 14 are aligned and mated so that the bondingmaterial on the capping wafer 14 contacts the bonding surface 26 of thedevice wafer 12. The wafers 12 and 14 are then heated to a temperaturesufficient to remelt the glass material but not the filler material ofthe bonding material, e.g., about 420° C., during which time force isapplied to the wafers 12 and 14 to cause a portion of the material toflow into the trench 34 of the capping wafer 14, and simultaneously asecond portion of the material flows beyond the peripheral edge 34. Theremainder of the glass bonding material remains between the bondingsurfaces 26 and 28 of the wafers 12 and 14. At this time, the bondingsurfaces 26 and 28 of the wafers 12 and 14 remain spaced apart by thestand-off distance established by the particle size of the fillermaterial, which does not melt during the bonding operation. Finally, thewafers 12 and 14 are cooled to resolidify the bonding material and formthe glass bond line 30 shown between the wafers 12 and 14 in FIG. 1.According to the invention, the filler material remains dispersed in theglass bond line 30 to establish the desired stand-off distance betweenthe bonding surfaces 26 and 28 of the wafers 12 and 14.

In FIG. 1, the portions of the glass bond line 30 that were forced outbetween the bonding surfaces 26 and 28 are accommodated within the openvolume alongside the edge 32 of the wafer 14 and a storage area definedby the trench 34 in the surface of the wafer 14. In effect, the edge 32and trench 34 define storage volumes for the molten glass materialdisplaced during the bonding operation. The significance of the slopingedge and trench walls 36 and 38 is related to the desire to minimize thewidth of the glass bond line 30, which in turn allows the chip size tobe reduced and, consequently, enables more device chips to be fabricatedin a single wafer slice. The desire to establish the stand-off distancebetween the wafers 12 and 14 by the diametrical dimension of theparticulate filler material results in the dimensions of thescreen-printed thick film paste being at the lower limits of the screenprinting process, with the result that variability in the thickness andvolume of the thick film paste is higher than what can be permitted toproduce a reliable and preferably hermetic glass bond. This variabilityis rectified by providing the storage volumes defined by the edge 32 andtrench 34 of the capping wafer 14 for the variable amount of moltenglass material that is displaced from between the bonding surfaces 26and 28 during the bonding operation.

As also seen in FIG. 1, the displaced portions of the bond line 30acquire a triangular-shaped cross-section as a result of the moltenglass material following the sloping surfaces of the walls 36 and 38. Inan investigation leading to this invention, the displaced portions ofthe bond line 30 roughly formed isosceles triangles, the cross-sectionalarea of which can be predicted for a given wall slope. Assuming astand-off distance of about 5.5 micrometers and the use of 1,0,0 siliconmaterial anisotropically etched to form the walls 36 and 38 at 54.7degree angles to the bonding surface 28, the triangular-shaped displacedportions of the bond line 30 have a combined volume that is greater thanthe remainder of the bond line 30 between the bonding surfaces 26 and 28if the horizontal offset of each wall 36 and 38 (one-half the base ofthe triangle) is more than 50 micrometers. Consequently, the slopingperipheral edge 32 and trench 34 of the capping wafer 14 effectivelyaccommodate excess molten glass during the bonding process, and provideefficient storage areas for the excess material of the bond line 30. Thewidth of the bonding surface 28, and therefore the distance between theedge 32 and trench 34 of the capping wafer 14, is preferably selected bydetermining the minimum required area for the bonding surface 28 basedon screen printing and bond strength requirements. In practice, asuitable width for the bonding surface 28 is about 300 to about 450micrometers, though narrower and wider widths are foreseeable.

While the edge 32 is shown as the outermost extremity of the cappingwafer 14 and the trench 34 is shown as also delineating the cavity 18within the wafer 14, the sloping walls 36 and 38 could be formed withtrenches whose sole function is to create the open volumes necessary toaccommodate the displaced portions of the bond line 30. Notably, propertrench design includes the ability to prevent air from becoming trappedas the molten glass fills the trenches. For example, the trench 34 isterminated outside, and not under, the bond line 30, to provide a paththrough which air is able to flow out of the trench 34 as the trench 34is filled with molten glass.

Referring again to FIG. 2, the runners 22 and isolation trenches 24present on the device wafer 12 complicate the ability to hermeticallyseal the cavity 18 of the package 10. An important aspect of the presentinvention is the ability to insure adequate flow of molten glassmaterial around the runners 22 and into the trenches 24 as a result ofthe glass bonding material “stored” in the storage areas defined by thetrench 34 and edge 32, and also in part by the preferred reflow stepprior to wafer alignment and bonding. As with the trenches 34, theisolation trenches 24 are preferably fabricated so that air will escapefrom the trenches 24 during bonding. The location and size of thestorage volumes defined by the trench 34, edge 32 and their walls 36 and38 must be sufficient to accommodate an excess amount of molten glassthat can fill the trenches 24 without depleting the molten glass betweenthe bonding surfaces 26 and 28. Consequently, the present inventionenables the isolation trenches 24 to be filled without resorting to anyother trench fill process, such as additional polysilicon growth, screenprinting, etc., often required in the past.

In view of the above, the present invention provides a glass fritbonding process by which the size of a device package, such as a MEMSdevice, can be minimized by eliminating the use of a discrete structuralfeature to space the wafers apart, and by reducing the bond line widthto something closer to the minimum width necessary to effect an airtightseal of adequate strength. In addition, the invention provides thecapability of forming a hermetic seal across trenches, runners and othersurface discontinuities without any additional and costly trench-fillprocess. While the invention has been described in terms of preferredaspects, features and materials, it is apparent that other forms couldbe adopted by one skilled in the art. Accordingly, the scope of theinvention is to be limited only by the following claims.

What is claimed is:
 1. A method of bonding first and secondsemiconductor wafers together so that contact surfaces of the first andsecond wafers are spaced apart by a stand-off distance, the methodcomprising the steps of: providing a mixture of a glass frit materialand a particulate filler material having a diametrical dimensionapproximately equal to the stand-off distance; depositing the mixture onthe contact surface of the first wafer so as to define a bond linethereon; mating the first and second wafers so that the bond line isbetween and contacts the contact surfaces of both of the first andsecond wafers; heating the mixture to melt the glass frit material andnot the particulate filler material so that the contact surfaces of thefirst and second wafers are spaced apart by the stand-off distanceestablished by the diametrical dimension of the particulate fillermaterial; and then cooling the glass frit material to form a glass bondline between the first and second wafers, the contact surfaces of thefirst and second wafers remaining approximately spaced apart by thestand-off distance.
 2. A method according to claim 1, wherein the firstand second wafers are a capping wafer and a device wafer, respectively,the device wafer having a micromachined element on a surface thereofthat is enclosed within a cavity defined by and between the device andcapping wafers.
 3. A method according to claim 1, wherein the glass fritmaterial is comprised of particles having a diametrical dimension thatis less than the diametrical dimension of the particulate fillermaterial.
 4. A method according to claim 1, wherein the first and secondwafers define a cavity therebetween following the mating step, themethod further comprising the step of forming a storage area in thefirst wafer that is spaced apart from and continuous around the cavityafter the first and second wafers are mated, the mixture being printedon the contact surface of the first wafer adjacent the storage area sothat a portion of the mixture flows into the storage area and not intothe cavity as a result of the mating step.
 5. A method according toclaim 4, wherein the mixture printed on the contact surface of the firstwafer is also adjacent a peripheral edge of the first wafer so that aportion of the mixture flows beyond the peripheral edge as a result ofthe mating step.
 6. A method according to claim 5, wherein the storagearea and the peripheral edge of the first wafer are defined byrespective walls contiguous with the contact surface of the first wafer,and wherein the walls are oblique to the contact surface of the firstwafer such that each of the portions of the mixture that flow into thestorage area and beyond the peripheral edge has a triangularcross-section after the mating step.
 7. A method according to claim 5,wherein the portions of the mixture have a greater volume than aremainder of the mixture between the storage area and the peripheraledge.
 8. A method according to claim 1, further comprising the steps ofheating the mixture after the depositing step so as to melt the glassfrit material but not the particulate filler material such that the bondline is composed of the particulate filler material dispersed in moltenglass, and then cooling the molten glass so as to solidify the bond lineprior to the mating step.
 9. A method according to claim 1, wherein thedepositing step comprises screen printing the mixture on the contactsurface of the first wafer.
 10. A method according to claim 1, wherein atrench is present in the contact surface of the second wafer, the bondline formed by the mixture crosses the trench, and the glass bond lineforms a hermetic seal between the first and second wafers.
 11. A methodaccording to claim 1, wherein the diametrical dimension of theparticulate filler material is about four to about six micrometers. 12.A method of bonding a capping wafer to a device wafer so that bondingsurfaces of the capping and device wafers are spaced apart by astand-off distance and the capping and device wafers form a package thatencloses a micromachined device formed on the device wafer, the methodcomprising the steps of: forming the capping wafer to have a peripheraledge and a trench in a surface thereof the trench being spaced inwardlyfrom the peripheral edge so as to define the bonding surface of thecapping wafer therebetween, the bonding surface of the capping waferbeing continuous so that the bonding surface will surround themicromachined device when the capping wafer is bonded to the devicewafer to form the package and enclose the micromachined device of thedevice wafer; providing a glass frit paste comprising a binder, asolvent, a glass frit material, and a particulate filler material, theparticulate filler material having a diametrical dimension approximatelyequal to the stand-off distance and having a higher melting temperaturethan the glass fit material; screen printing the glass frit paste on thebonding surface of the capping wafer so that the glass fit paste definesa bond line on the bonding surface; heating the glass frit paste toremove the binder and the solvent and melt the glass frit paste so thatthe bond line comprises the particulate filler material in a moltenglass material; resolidifying the bond line so that the particulatefiller material remains dispersed in a solid glass material; mating thedevice and capping wafers so that the bond line is between and contactsthe bonding surfaces of the capping and device wafers; heating thedevice and capping wafers to melt the solid glass material but not theparticulate filler material, a first portion of the bond line flowinginto the wench and a second portion of the bond line flowing beyond theperipheral edge while a remainder of the bond line remains between thebonding surfaces of the device and capping wafers, the bonding surfacesof the device and capping wafers being spaced apart by the stand-offdistance established by the diametrical dimension of the particulatefiller material; and then cooling the device and capping wafers toresolidify the bond line and form a glass bond line between the deviceand capping wafers, the bonding surfaces of the device and cappingwafers remaining approximately spaced apart by the stand-off distance.13. A method according to claim 12, wherein the trench and theperipheral edge of the capping wafer are defined by respective wallscontiguous with the bonding surface of the capping wafer, and whereinthe walls are oblique to the bonding surface of the capping wafer.
 14. Amethod according to claim 13, wherein each of the first and secondportions of the bond line that flow into the trench and beyond theperipheral edge has a triangular cross-section after the step of heatingthe device and capping wafers to remelt the bond line.
 15. A methodaccording to claim 12, wherein the first and second portions of the bondline have a combined volume that is greater than the remainder of thebond line between the bonding surfaces of the device and capping wafers.16. A method according to claim 12, wherein an isolation trench ispresent in the bonding surface of the device wafer, the bond linecrosses the isolation trench, and the glass bond line forms a hermeticseal between the device and capping wafers.
 17. A method according toclaim 12, wherein the diametrical dimension of the particulate fillermaterial is about four to about six micrometers.
 18. A method accordingto claim 12, wherein the bond line following the resolidification stepand the glass bond line following the cooling step are substantiallyfree of voids.